Time Slot Interchange (TSI) facilities are used in time division multiplex switching systems to interconnct a calling station served by a first system time slot to a called station served by a second system time slot. This permits a common signal path to serve concurrently a plurality of calls by time sharing the use of the common path. Each call connection is granted exclusive use of the path for time interval defined by the system time slots associated with the connection.
A basic version of a time slot interchanger comprises a time slot driven RAM which is written with call information during a first system time slot under control of a first source of address information and which is read out during a second system time slot under control of a second source of address information. Each system time slot is associated with a unique RAM location as well as with a unique station served by the system. PCM encoded signals generated at a first station on a call are written into the associated input RAM location during each occurrence of the time slot associated with the first station. The system's time slot counter is used at that time as the addressing signal source for the RAM.
This PCM call information is transferred to a second station involved on the call during each occurrence of the system time slot assigned to the second station. This transfer is effected by applying the output of the system time slot counter to a translation RAM which generates an output signal identifying the time slot serving the first station. This generated time slot number is applied as addressing information to the time slot driven RAM which leads out the call information for the first station from its addressed location and applies the readout information over time shared PCM paths extending to the second station.
U.S. Pat. No. 4,112,258 issued Sept. 5, 1978 to H. G. Alles discloses an improved time slot interchanger that comprises a program controlled signal processor. In addition to performing a conventional time slot interchange function, the Alles TSI can insert a specified gain or loss into the interchange of any call signal. The Alles TSI also provides conferencing facilities. These facilities include the use of a single register operating as an accumulator for generating and storing the conferee sum and difference signals required in the serving of conference calls.
In Alles, all call data are written into an input RAM during each time frame. These data are next transferred to a processor of the TSI for processing. The processed call data are then transferred to an output RAM and are subsequently readout of the output RAM during another time frame. The time required to transfer data from the TSI input RAM via the TSI processor to the TSI output RAM comprises reading time, processing time, and writing time. The number of call connections that can be served as a TSI of the Alles type is limited by the processing time required to convey a signal from the input RAM to the output RAM.
A certain minimum number of PCM samples per second must be transferred between connected stations to maintain the required system frequency response of a PCM system. Since all time slots are served only once each time frame, a minimum sampling rate must be maintained for the number of frames transferred per second from the input RAM to the output RAM. This is called the system frame rate. The TSI input and output RAMs can read and write data at a faster rate than the Alles TSI processor can process the data. The processing time determines the number of call connections that can be served within the system frame time. Without changing the system's frame rate, the Alles TSI RAMs could be read and written at a faster rate if the processing time could be reduced. This would increase the call serving capacity of the system.